The present invention relates generally to a display device for an automotive vehicle for displaying vehicular driving information, such as vehicle speed, engine speed, fuel supply, engine coolant temperature and so forth. More specifically, the invention relates to a compact projector device which is designed for use in a so-called "head-up display" which projects vehicle driving information onto the front windshield.
Conventionally, vehicular instruments, such as speedometers, trip meters, odometers, fuel gauges, temperature meters, clocks and so forth, have been arranged on an instrument panel. This arrangement of instruments has forced the driver to look down and/or away from the road to obtain necessary driving information. This incurs an interruption in the observation of the surrounding terrain and traffic.
In order to improve this, there has been a so-called "head-up display" has been proposed which focuses images of a speedometer, a trip meter, an odometer, a fuel gauge, a temperature meter, a clock and so forth to overlap the forward field of vision through the windshield. This display system is disclosed in Japanese Patent First Publication (Tokkai) Showa No. 57-182541. The display system employs a cathode-ray tube (CRT) as a source of display images. The CRT is rather bulky due largely to its thickness. Furthermore, in order to improve the legibility and sharpness of the projected image, a rather high luminosity is needed, which further adds to its bulk. A bulky CRT is difficult to install within the instrument panel.
The head-up display system further requires an optical system for focussing the display image onto the front windshield. Relatively thick and bulky lenses must be used in the optical system. This bulky optical system again limits where the head-up display system can be installed due to its weight and bulk.
Improved head-up display systems proposed in Japanese Utility Model First Publication (Jikkais) Showa Nos. 58-180522 and 59-7428 help alleviate these difficulties with the prior art. These systems employ holograms as combiners. However, since such systems still require CRT's and optical systems, they cannot completely solve the aforementioned problems.
Because of the problems in the prior proposed systems, there is no head-up display system which can be installed practically in a commercially available vehicle.